1. Field of the Invention
The present invention relates to an extrusion method for manufacturing tread bands for vehicle tires, as well as an extrusion apparatus for carrying said method into effect, said apparatus being of the type comprising: at least one primary extrusion head for feeding at least one primary elastomer material; at least one auxiliary extrusion head for feeding at least one auxiliary elastomer material; a preforming device having at least one primary preforming channel connected to the primary extrusion head; an extrusion die operatively disposed downstream of the preforming device and having an extrusion outlet located at the end of the primary preforming channel and defining an extrusion path therewith.
2. Description of the Related Art
It is known that in the field of tire manufacture for vehicle wheels, the tread band is obtained by extrusion of a workpiece in the form of a continuous strip which is subsequently cut into lengths of predetermined measure, each of which is adapted to be associated with the other tire components during an assembling step provided in the production process: during this step all workpieces forming the tire are fit together at room temperature, said pieces being cold too, since they come from a storage magazine.
In more detail, the stretch forming the tread band is applied crownwise either to the carcass, usually already toroidally shaped, directly on the carcass ply or plies, or to the belt structure previously wrapped over said carcass, in such a manner that the opposite side edges of the tread band join the end flaps of the tire sidewalls. Depending on planning choices, these end flaps are either applied to the carcass plies so as to be enclosed between the latter and the opposite side edges of the tread band, or directly applied in superposed relationship to the opposite side edges of the tread band itself.
Usually, the type of blend used in making the tread band depending on the required performance (wear resistance and grip on a dry and wet road surface, for example) has such chemical features that it appears difficult to obtain an efficient cold adhesion to the elastomers employed in making the carcass and belt structures and the tire sidewalls. Under this situation, in order to eliminate the necessity to submit the tread band to processes involving application of a petrol solution in order to restore the necessary stickiness during the tire assembling step, in many cases use is made of a so-called “skim coat” to be applied to the lower surface of the tread band and intended to get into contact with the carcass ply or belt structure. This skim coat is essentially made up of a thin layer of natural rubber-based blend that, being applied to the tread band when it is still hot immediately after its emerging from the extrusion head, can be efficiently anchored to the tread band blend and supplies an appropriate substrate for cold adhesion to the carcass without resorting to surface processes involving application of a petrol solution.
Likewise, in the case of overlapping of the sidewalls on the opposite side edges of the tread band, the side edges of the tread band as well are intended to be provided with two border inserts, the so-called “mini-sidewalls” also made of an elastomer material having such features that an efficient adhesion between tread band and sidewalls is ensured during the assembling step of the tire, usually by adopting the same blend for the sidewalls and mini-sidewalls in order to facilitate cold adhesion.
Application of mini-sidewalls to the tread band takes place by a co-extrusion process, a hot process therefore, which consists in employing on the same extrusion unit intended for the tread band formation, a second extrusion head supplying the blend for the mini-sidewalls along two auxiliary feeding channels converging at the opposite side portions of an extrusion outlet, in a direction substantially parallel to the course of the primary feeding channel through which the elastomer material intended for forming the true tread band is fed.
In the present state of the art, unlike the mini-sidewalls, the skim coat is produced in a separated unit, generally a calender, and rolled onto bobbins; each bobbin is then mounted on an appropriate unwinding device from which the skim coat is taken out for application to the tread band already provided with the optional mini-sidewalls, while said tread band is moving away from the corresponding extrusion unit.
Usually, since the skim coat is applied to the lower surface of the tread band, the unit carrying out unwinding of the skim coat is disposed under the whole tread band extrusion plant that consequently must be placed at a raised position, which will bring about complications in the plant structure and difficulties in acceding to all parts thereof.
In order to eliminate the requirement for use of a separated unit for skim coat production and a bobbin unwinding unit to deliver the skim coat to the tread band extrusion unit, attempts have been made to extrude the skim coat together with the tread band, in the same manner as pointed out with reference to mini-sidewalls, by adopting a co-extrusion method already known and successfully used for producing tread bands made up of two blends, where thickness of the lower blend layer is about half the thickness of the whole tread band.
The Applicant has however found that this co-extrusion method gives poor results when, in an attempt to make the skim coat, thickness of the underlying blend layer is reduced to values as low as two millimeters or less. Actually, in order to obtain very reduced thicknesses by extrusion, of 2 mm for example, due to swelling of the blend on coming out of the extrusion device, dies having apertures of about 1 mm are to be adopted and consequently possible blend impurities or dust may be entrapped into the die, thereby creating longitudinal scratches and therefore size irregularities in the extruded product or extrusion.
In addition, under such operating conditions checking of the blend flow rates at the different points of the extrusion section is very difficult and, therefore, sufficient assurances in terms of even and precise thickness of the skim coat and repeatability of results can be hardly given.
It is known that these difficulties are not present in calenders.